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EFFECT OF DISPLACEMENT OF HEART DURING OBTUSE MARGINAL GRAFTING IN OFF-PUMP CARDIAC SURGERY ON OXYGENATION-AN OBSERVATIONAL STUDY.

Karthekeyan et al

ABSTRACT

Objective: To study the effect of displacement of heart during Obtuse Marginal grafting in off-pump coronary artery bypass on partial pressure of oxygen in arterial (PaO2).
Design: Prospective
Setting: Tertiary teaching institute
Participants: 50 consecutive patients
Measurements and Main Results: All patients were provided general endotracheal anaesthesia. After median sternotomy, heart was positioned with Octopus Tissue Stabilizer. Partial pressure of oxygen in arterial blood (PaO2) and mixed venous blood (MvO2), just before positioning, 5 minutes after positioning and 5 minutes after end of grafting, were collected. The data were analysed using student’s paired t test. PaO2 and MvO2 showed a significant reduction 5 minutes after positioning when compared to the baseline (27.566 ± 59.777 mm of Hg and 3.242 6 ± 3.812 mm of Hg respectively).
Conclusions: Positioning during obtuse marginal grafting in OPCAB produces significant reduction in PaO2 and MvO2.

KEYWORDS

Oxygenation;Obtuse marginal;Mixed venous;Arterial oxygen;Off-pump

Introduction

Optimal exposure and stabilization of the target coronary vessel is essential for precise coronary anastomosis during off pump coronary artery bypass (OPCAB) surgeries. This might be achieved at the expense of significant haemodynamic deterioration, particularly while grafting the obtuse marginal artery. While emphasis is laid on hemodynamics, there are not many studies on oxygenation status in off pump coronary bypass surgery. This study was designed to assess the oxygenation status of the patient during grafting of obtuse marginal (OM) artery in OPCABs.

Materials and methods

After getting approval from the Institutional ethics committee, 50 consecutive patients undergoing OPCAB were included in the study. The exclusion criteria included history of smoking, occupational lung diseases, bronchial asthma, breath holding time less than 40 seconds and history of any other respiratory illness. Informed written consent was obtained from all the patients. The patients were fasted for 6 hours preoperatively and premedicated with tablet diazepam and tablet ranitidine the night before and on the morning of the day of the surgery. Preinduction monitors included pulse oximeter(SpO2), electrocardiogram (lead II and V5 with ST analysis), invasive arterial blood pressure through radial artery cannulation, pulmonary artery catheter and Bi Spectoral Index (A – 2000TM BISTM). Induction of anaesthesia was with titrated doses of injection midazolam, fentanyl and thiopentone. Neuromuscular blockade was achieved with injection vecuronium. After 180 seconds of bag and mask ventilation with 100% oxygen, trachea was intubated with appropriate sized cuffed endotraceal tube. Post induction monitoring included end tidal carbon dioxide, nasopharyngeal temperature, urine output and femoral artery pressure. Anaesthesia was maintained with nitrous oxide in oxygen (50%), sevoflurane, midazolam and fentanyl, titrated to a BIS value of 40 to 60. Temperature was maintained between 34 to 36 degrees. All patients underwent median sternotomy. Heart was positioned with coronary stabilizer (Octopus Tissue Stabilizer, Medtronic, Inc, Minneapolis, MN). Arterial blood samples and mixed venous samples just before positioning (baseline), 5 minutes after positioning and 5 minutes after end of grafting were collected and corresponding SpO2 was noted. The hemodynamics of the patients were maintained with fluids, injection ephedrine 6 mg boluses and Trendelenberg positioning.
Statistical Analysis
Data are presented as mean ± standard deviation (absolute values). For comparison of baseline data with the data obtained during heart displacement, a paired Student’s t test was used. A 'p value' less than 0.05 was considered statistically significant.

Results

The age distribution is shown in table 1. The patients ranged from 36 years to 78 years and all were males.
The mean PaO2 and MvO2 at the three study periods have been shown in table.2. The mean reduction in PaO2 and MvO2 at the study periods in comparison with the baseline have been shown in tables 3.

Table.1 : Patient characteristics.

Variables

Age in years

Mean - 60.5

( range 36 to 78)

Table.2: Mean PaO2 and MvO2 at three study periods a

Time of observation	PaO2 (mm of Hg)	MvO2 (mm of Hg)
Baseline	197.466 ± 73.848	41.806 ± 4.701
5 minutes after positioning	169.9 ± 65.947	38.564 ± 4.115
5 minutes after grafting	184.98 ± 71.794	42.144 ± 4.505

a Values are shown as mean ± standard deviation

Table.3: Mean difference in PaO2 and MvO2 at three study periods

Difference in

PaO2 (mm of Hg)a

p value

Difference in

MvO2 (mm of Hg)a

p value

(Baseline) - (5 minutes after positioning)

27.566 ± 59.777

0.002*

3.242 6 ± 3.812

0.001*

(Baseline) -

(5 minutes after completion of grafting)

12.486 ± 63.418

0.170**

0.338 ± 3.761

0.528**

(5 minutes after positioning) - (5 minutes after completion of grafting)

15.080 ± 45.693

0.024*

3.580 ± 3.414

0.002*

a Values are shown as mean ± standard deviation. * - p < 0.05, ** - p > 0.05

PaO2 showed a significant reduction (p = 0.002), from a baseline mean value of 197.466 ± 73.848 mm of Hg to a mean value 169.9 ± 65.947 mm of Hg, after 5 minutes of positioning. However PaO2 after 5 minutes of completion of grafting was not significantly less when compared to the baseline (p = 0.17). MvO2 also showed a similar significant reduction from 41.806 ± 4.701 mm of Hg at baseline to 38.564 ± 4.115 mm of Hg after 5 minutes of positioning (p = 0.001). But the difference in MvO2 between baseline and the values at 5 minutes after completion of grafting was not significant (p = 0.528).
Among the 50 patients, two had a decrease only in PaO2 while the MvO2 did not show a reduction. The first among the two had a reduction in PaO2 from a baseline of 184.2 mm of Hg to 163.5 mm of Hg 5 minutes after positioning, while the MvO2 had only a slight change from 37.5 mm of Hg at baseline to 37.6 mm of Hg 5 minutes after positioning. The second patient also had a reduction in PaO2 from 307.8 mm of Hg at baseline to 301.9 mm of Hg after 5 minutes of positioning while the MvO2 changed from a baseline of 37.3 mm of Hg to 37.7 mm of Hg at 5 minutes after positioning.
Discussion:
In our study, there was a significant reduction in partial pressure of oxygen in arterial blood (PaO2) and partial pressure of oxygen in mixed venous blood (MvO2), after positioning for obtuse marginal grafting. This decrease was reversed after repositioning, indicating that this reduction is due to positioning. This reduction in oxygenation, along with the decrease in cardiac output and coronary blood flow1 after positioning, can contribute to decreased oxygen delivery to the myocardium.
Arno P N and others in 20002 have shown that the right ventricle gets squeezed between the pericardium and the left ventricle, during the obtuse marginal grafting, creating an acute low cardiac output state. Unlike other low cardiac output conditions, here left ventricular filling pressures decreased. They have shown a significant reduction SvO2 during OM grafting. The right ventricular outflow gets distorted during OM grafting. We have shown a significant reduction in MvO2 during OM grafting and one of the possible causes for this might be due to decrease pulmonary blood flow due to this distortion and low right ventricular output.
Mitral annulus distortion with enlargement of left atrium and pulmonary veins have been shown to occur during grafting in beating heart surgeries3. This engorgement of pulmonary veins may cause congestion of pulmonary bed accounting for low oxygenation. Thus not only underflow but overflow in the pulmonary bed also can account for decreased oxygenation during the position for OM grafting.
A reduction in cardiac output could have also contributed to the decrease in MvO2. But it has been shown that MvO2 poorly correlates with the cardiac index4. In addition, two of the patients had a decrease in PaO2 without a concomitant decrease in MvO2. Arno P N and others in 20002, have also shown bulging of interatrial septum to the left side by trans esophageal echocardiogram during OM grafting. Megumi and co workers in 2000 have reported that the stabilizers compress the right ventricular cavity just below the tricuspid valve and the right atrium enlarges due to this compression5. In this situation, a preexisting foramen ovale could cause a right to left shunt, which may account for decreased oxygenation as seen in our study, by a mechanism similar to that produced by positive end expiratory pressure6.
LIMITATIONS:
Though we could demonstrate the decrease in oxygenation we could not clearly find the etiology except in one case where a PFO was demonstrated. The decreased MvO2 cannot be taken as a surrogate for low cardiac output as they are many other confounding factors. We did not study the oxygenation of the myocardium with coronary lactate levels and the implications of decreased oxygenation are not discussed. The positioning for LAD and PDA grafting may produce similar but less dramatic decrease in oxygenation which is to be studied.

Conclusion

It is evident that there is decrease in arterial oxygen tension during OPCAB, especially during OM grafting. Since there is a co existing decrease in MvO2, low cardiac output can be proposed as a possible cause. But since a decrease in PaO2 can inturn produce a decrease in MvO2, other factors like patent foramen ovale, pulmonary venous obstruction, collection of fluid in the pleura should also be explored.

References

Paul F Grundeman, Cornelius B, Joost A van Herwaarden et al. Vertical Displacement of the Beating Heart by the Octopus Tissue Stabilizer: Influence on Coronary Flow. Ann Thorac Surg 1998;65:1348-1352

Arno P. Nierich, Jan Diephuis, Erik W.L. Jansen et al. Heart displacement during off-pump CABG: how well is it tolerated? - Ann Thorac Surg ; 2000; 70:466-472

Shane J. George, Sharif Al-Ruzzeh and Mohamed Amrani. Mitral annulus distortion during beating heart surgery: a potential cause for hemodynamic disturbance—a three-dimensional echocardiography reconstruction study. Ann Thorac Surg 2002;73:1424-1430

Sommers MS, Stevenson JS, Hamlin RL et al. Mixed venous oxygen saturation and oxygen partial pressure as predictors of cardiac index after coronary artery bypass grafting. Heart Lung 1993; 22(2):112-20

Megumi M, James R. Edgerton, Jeffrey L. Horswell et al. Analysis of Hemodynamic Changes During Beating Heart Surgical Procedures. Ann Thorac Surg 2000;70:1355-1360

Papadopoulos G, Brock M, Eyrich K. Intraoperative contrast echocardiography for detection of a patient foramen ovale using a provocation test and ventilation with PEEP respiration. Anaesthesist. 1996 Mar;45(3):235-9

Authors

1. Ranjith B Karthekeyan; MD, DNB
Associate Professor,
Dept of Cardiac anesthesiology,
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India.

2. Karthikeyan N Selvaraju; MD,
Resident, Dept of Cardiac anesthesiology,
Sri Ramachandra Medical College and research Institute, Porur,
Chennai, India.

3. Ramesh MD
Resident, Dept of Cardiac anesthesiology,
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India.

4. Suresh Rao K G; MD
Prof and chief, Dept of Cardiac anesthesiology,
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India

5. Mahesh Vakamudi; MD
Prof and Head, Dept of anesthesiology and critical care,
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India.

6. Balakrishnan K R; MS, Mch
Prof and Head, Dept of Cardiothoracic and vascular surgery
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India

7. Harish MD
Resident, Dept of Cardiac anesthesiology,
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India

8. Dr.Siva Muthukumar
Assistant Professor,
Dept of Cardiac anesthesiology,
Sri Ramachandra Medical College and research Institute, Porur, Chennai, India.

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